The invention relates to an improvement in the tail threading method used in paper and board machines and to a finishing device for paper, especially a multi-roll calender, placed in a paper or board machine or in an aftertreatment machine of paper or board and applying the improved tail threading method.
The FI patent 62874 of the applicant discloses a calender for paper or the like, which is intended to be integrated in a paper or board machine to calender the web passed from the same. The calender utilizes a threading method, which comprises two endless ropes which are arranged to travel in the vicinity of one vertical frame column of the calender guided by rope sheaves and via grooves located on the mantles of the calender rolls. According to the specification, the threading takes place in such a way that the calender nips are open and the calender rolls rotate at a speed corresponding to the feed rate of the web in the paper machine, i.e. they are driven. A narrow strip of the web passed from the drying section of the paper machine is guided into the gap or throat between the ropes at the stage when the ropes are guided to the rope groove of the roll. This so-called tail travels between the ropes and follows their path as they are winding via the calender rolls in accordance with the travel path of the web according to the function of the calender.
Futhermore, a slightly more modem calender concept which can be placed within the paper machine, is disclosed in the FI patent 96334 by the applicant, which describes a multi-roll calender provided with modem roll coatings, and applying rolls with both hard and soft surfaces. Naturally, when placing the calender according to this invention within the paper machine, it should include for example a threading system according to the FI patent 62874 which is capable of performing threading when the web runs at the driving speed of the paper machine, which can even exceed the value of 2000 m/min.
In such a threading method, the tail does not travel along a straight path but winds i.e. bends in directions perpendicular to its plane, in such a way that the transverse stiffness of the tail ceases to affect the travel of the tail. Thus even the weakest lateral forces that affect the tail can move the tail in the lateral direction during threading. The characteristics of the tail can be compared to a flexible lath which, when in a straight position, functions like a bending beam when being subjected to lateral forces, but when it is placed in a bent position, its stiffness which still resists the transverse force, is due to the torsional stiffness effective in the cross-sectional plane of the lath, wherein a considerably weaker transverse force is required to deflect the end of the lath when compared to a plain bending situation.
The lateral forces effective on the tail in the paper machine are produced for example by currents of air developed in the paper machine by friction between the surfaces of rapidly rotating rolls and the surrounding air, and by the different temperatures of various machine sections, as well as by the differences in the temperatures of the machine sections and ambient air.
It has been observed that the air flows tend to move the web off the machine in the cross machine direction, especially in the vicinity of such points where two rolls rotating in opposite directions are located close to each other. An example that can be mentioned is a calender nip which is open during threading.
The tail is separated from the full-sized web in a suitable location in the machine section preceding the calender, for example in the drying section, with a cutting means which separates a strip of a particular width from the edge of the web. The forward end of this strip is guided between continuous threading means, such as ropes, belts, or the like, in order to draw the tail even long stretches in the future travel path of the web. Thus, a problem in the threading methods of prior art is that when the tail is passed by means of ropes, belts, or the like, through the calender or a corresponding paper processing device, the tail following the forward end placed in the threading means tends to wander onto the same line with the threading means again: underneath, above or between the same, in the travel direction of the tail behind the actual draw point where it should settle on the correct route for example between the rolls. The tail breaks easily at this point and the threading attempt has to be initiated again.
When this strip which runs freely and in a relatively uncontrolled manner in the end section of the tail, ends up on the same line with the threading means, for example on top of them, the strip may still be located on the top i.e. in a free position on the next reversing roll, but thereafter on the next one, the strip may already be located between the threading means and their guide means, i.e. belt or rope sheaves. The belt or rope sheaves or the other guiding devices of the threading means are provided with such a structure that their outer diameter corresponds to the outer diameter of the roll by which they are placed in the bend or reversing point of the path and along which the tail travels, and they contain a groove whose bottom, however, has a smaller diameter than the diameter of the corresponding roll. Therefore, the speed of the tail is reduced when it is ends up between the threading means and its guiding device, and as a result, the tail is tightened and breaks because the actual forward end of the tail is running at the surface speed of the rolls. Correspondingly, the tail is slackened in the travel direction of the web after the aforementioned disturbance point, thus causing an increasing tendency of the tail to wander as the tightness is lost.
Due to the above-described phenomena, it is most likely that the tail will break in this disturbance situation.
U.S. Pat. No. 5,234,549 discloses a so-called dual cutter comprising two cutting blades, intended for separating the tail from the web. By means of this dual cutter it is possible to affect the location of the tail in the cross direction of the machine by moving the blade separating the tail from the wide web closer to the centre, and by cutting a narrow strip from the wide strip so obtained at its outer edge. The edge strip to be conveyed to the calender moves in this way closer to the centre in the cross-machine direction. Thus, this arrangement requires two independently movable cutting blades.
It is an objective of the invention to produce a threading method and a finishing device for paper, by means of which it is possible to prevent the tail from breaking due to the aforementioned reasons during threading. Furthermore, an objective of the invention is to produce a threading method and a finishing device for paper whereby it is possible to prevent the tail from wandering outwards with respect to the centre line of the machine during tail transfer before spreading the web to its full width, also in cases where only a single cutter is available, i.e. in conventional threading operations where a tail narrower than the rest of the paper or board web is separated from the outer edge of the web along a single cutting line.